Westie Jaw

Issue DescriptionA developmental disease in dogs causing
extensive bony changes in the mandible and skull. In this disease
there is a cyclical resorption of normal bone and replacement by
immature bone along the inner and outer surfaces of the affected
bones. It usually occurs between the ages of 3 and 8 months. Breeds
most commonly affected include the West Highland White Terrier,
Scottish Terrier, Cairn Terrier, and Boston Terrier. It is rare in
large breed dogs, but it has been reported.Other NamesCMO, Mandibular Periostitis, Temporomandibular
Osteodystrophy, Lion's Jaw, Craniomandibular Osteopathy, Scottie Jaw

SymptomsSymptoms include firm swelling of the jaw,
drooling, pain, and difficulty eating. It is an inherited disease,
especially in Westies, in which it has been recognized as an autosomal
recessive trait. Canine distemper has also been indicated as a
possible cause, as has E. coli infection, which could be why it is
seen occasionally in large breed dogs. Growth of lesions will usually
stop around the age of one year, and possibly regress. This timing
coincides with the normal completion of endochondral bone growth and
ossification. If the disease is extensive, especially around the
tympanic bulla (middle ear), then the prognosis is guarded.

DiagnosisThe disease is most often diagnosed by clinical
signs and palpation with definitive confirmation by lateral and/or
ventral/dorsal radiographs of the skull, depending on the location of
the specific lesion. All board-certified radiologists can diagnose the
disease, as can many other experienced veterinarians.

TreatmentCMO is treatable. Depending on the severity,
the amount of medication and length of treatment vary. Four to ten
months is the average length of treatment. Many affected puppies will
require some dose of cortisone until they are 10 months or older. Most
anti-inflammatory drugs work well, but since CMO may require long-term
therapy, veterinary advice is essential. Puppies nearly always
recover. In severe cases, tube feeding may be required during appetite
loss to support nutrition. The disease can be controlled by
corticosteroid therapy and buffered aspirin (pain). When treated, the
bony swelling often recedes, but some enlargement and impaired chewing
may continue for the rest of the dog's life.

Affected dogs
should not be used for breeding, even if they appear to recover
completely. Breeding of parents (carriers) and siblings (suspect
carriers) should also be avoided. The Institute for Genetic Disease
Control in Animals (GDC) and the Orthopedic Foundation for Animals
(OFA) maintain registries for this condition in terrier breeds.

People seeking purebred terriers, especially West Highland white
terriers, should question breeders carefully about the occurrence of
the disorder in any lines, as CMO is inherited in Westies, and is
believed to be inherited in Scottish terriers as well. Optimal
treatment for your pet requires a combination of home and professional
veterinary care, with good rest on comfortable dog crates. Follow-up
can be critical, especially if your pet does not rapidly improve.
Administer all prescribed medication as directed. The disease often
stops progressing around 11 to 13 months of age, and then may regress
partially or completely.

Imaging

Radiographs of the skull show uneven,
bead-like osseous proliferation of the mandible or tympanic bullae
(bilateral); extensive, periosteal new bone formation (exostoses)
affecting one or more bones around the TMJ. Fusion of the tympanic
bullae and angular process of the mandible may occur.

CT scan may be useful in evaluating the osseous involvement of the
TMJ.

Other Diagnostic Procedures

Biopsy necessary only in atypical patients.

Bone biopsy reveals normal lamellar bone being replaced by an
enlarged coarse fiber bone and osteoclastic osteolysis of the
periosteal/subperiosteal region. The bone marrow is replaced by a
vascular, fibrous-type stroma. Inflammatory cells occasionally can be
seen at the periphery of the bony lesion.